In electrophotography, a latent image is created on the surface of photoconducting material by selectively exposing areas of the charged surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity, but different levels of charge.
A sheet of paper or intermediate transfer medium is then given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium, still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers fixes the toner to the paper, subsequent to direct transfer, or indirect transfer when using an intermediate transfer medium, producing the printed image.
The important photoconductor surface, therefore, has been the subject of much research and development in the electrophotography art. A large number of photoconductor materials have been disclosed as being suitable for the electrophotographic photoconductor surface. For example, inorganic compounds such as amorphous silicon (Si), arsenic selenite (As.sub.2 Se.sub.3), cadmium sulfide (CdS), selenium (Se), titanium oxide (TiO.sub.2) and zinc oxide (ZnO) function as photoconductors. However, these inorganic materials do not satisfy modern requirements in the electrophotography art of low production costs, high-speed response to laser diode or other light-emitting-diode (LED), and safety from non-toxicity.
Therefore, recent progress in the electrophotography art with the photoconductor surface has been made with organic materials as organic photoconductors (OPC's). Typically, the OPC's in the current market are of the negative-charging type with a thin charge generation material layer, usually less than about 1 micron (.mu.m) thick, beneath a thicker charge transport material layer deposited on top of the charge generation layer. The negative-charging OPC's perform well for xerographic copiers and printers in the following applications:
a. Low end (4-10 copies per minute) and high end (more than 50 copies per minute) xerographic systems using dry powder developers of one or two colors, or using liquid developers for black and white copies only; and,
b. High image quality (above 1800 DPI) color proofing, lithographic plate printing and master xerographic printing systems with life expectancies of less than 100 cycles.
However, prior art negative-charging OPC's also have several drawbacks, namely:
1. Large amounts of ozone are generated in the negative corona charging process, creating environmental concerns. This problem has been addressed by installing ozone absorbers like activated carbon filters, and by using contact negative charging instead of corona charging. These ozone remediation approaches, however, have drawbacks of their own and are not attractive commercial solutions.
2. Negative corona charging generally results in less charge pattern uniformity compared to positive corona charging. Lower charge pattern uniformity in turn results in more noise and less definition in the final image.
3. In small particle toner processes, including fine dry powder and liquid toner processes, designers have been able to develop more charge stability in positively charged toners than in negatively charged toners. Therefore, positive charging OPC's ((+)OPC's) are preferred for a discharged area developed image as in laser printers.
Specific morphologies of phthalocyanine pigment powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate. In these phthalocyanine/binder photoconductors, the photo-generation of charge and the charge transport occur in the particles of the phthalocyanine pigment while the binder is inert. Therefore, the photoconductor may be made of a single layer of phthalocyanine/binder. These single-layer photoconductors are known to be very good positive charging OPC's due to the hole (positive charge) transportability of the phthalocyanine pigment.
In these single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 10-30 wt. %, high enough to perform both charge generation and charge transport functions, with the binder content being in the range of about 90-70 wt. %. The single photoconductor layer is usually more than about 3 .mu.m thick in order to achieve the required charge acceptance and resulting image contrast.
Therefore, it is a first object of this invention to provide a (+)OPC which exhibits stable electrical properties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity electrophotographic process. Modern digital imaging systems, wherein the writing head is LED array or laser diode, have very high light intensities (about 100 ergs/cm.sup.2) over very short exposure time spans (less than 50 nano-seconds), resulting in severe conditions for the OPC compared to optical input copiers with light intensities between about 10-30 ergs/cm.sup.2 and exposure times between about several hundred microseconds to milliseconds.
Unfortunately, there is no product on the market today which provides such stable electrical properties. This is because the (+)OPC exhibits instability when it is frequently exposed to the corona charger and the intense light source in the electrophotographic process. I have discovered this instability to be more pronounced at the strong absorption, high light intensity, short exposure time conditions required for the laser printing process. The instability of the photoconductor is exhibited in the significant increase of its dark decay characteristic after a relatively small number of repeat cycles of laser printing. Also, the instability is exhibited in the decrease in surface potential after repeat cycles. These instabilities cause deleterious changes in image contrast, and raise the issue of the reliability of image quality.
Preferably, desirable electrophotographic performance may be defined as high charge acceptance of about 60-100 V/.mu.m, low dark decay of less than about 5 V/sec., and photodischarge of at least 90% of surface charge with the laser diode beam of 780 nm or 830 nm frequency, through the optical system including beam scanner and focus lenses, synchronized at 0.05 micro seconds for each beam.
When conventional binders for the phthalocyanine pigment, such as acrylic resins, phenoxy resins, vinyl polymers including polyvinyl acetate and polyvinyl butyral, polystyrene, polyesters, polyamides, polyimides, polycarbonates, methyl methacrylate, polysulfones, polyarylates, diallyl phthalate resins, polyethylenes and halogenated polymers, including polyvinyl chloride, polyfluorocarbon, etc., are used, acceptable charge acceptance and photodischarge are obtained. However, among these polymers which result in good performance for charge acceptance and photodischarge, none of them exhibit the desirable stability under the severe LED array or laser diode exposure conditions described above.
The conventional OPC's are presently made with thermoplastic binders which exhibit poor wear resistance, especially in high-speed, high-cycle applications using two-component developers, including magnetic carrier and toner, and in applications using tough cleaning blade materials such as polyurethane. Generally, an OPC with a mechanically worn surface exhibits diminished electrophotographic properties, such as low charge acceptance, high dark decay rate, low speed and low contrast.
A second object of this invention is to provide an OPC with superior durability from mechanical strength, solvent resistance and thermal stability. The OPC must be mechanically strong in order to ensure wear resistance in high cycle applications. It must be solvent resistant in order to prevent it from being changed or lost in the liquid toner applications. It must be thermally stable in order to ensure predictable and repeatable performance at and after different operating temperatures, especially the elevated temperatures, typically about 70.degree. C., for modern laser printers.
Also, the conventional thermoplastic binders exhibit higher solubility in the solvents used in liquid toner applications. For example, in the wet environment required to achieve very high resolution above 1200 DPI associated with high-end applications, the liquid carrier tends to partially dissolve the OPC's binder, causing diminished resolution. Also, in aqueous inking applications, water has an adverse effect on the conductivity of OPC's made with these conventional binders, which effect is aggravated by higher temperatures.
Also, the conventional thermoplastic binders exhibit high thermal degradation in the electrical properties important for electrophotography, reflected in decreased charge acceptance, increased dark decay rate and reduced contrast potential.
A third object of this invention is to provide a cross-linked binder for an OPC without having to provide also, besides the binder material, a cross-linker material, or a cross-linkable copolymer material, or a cross-linking catalyst, which may affect the life of the OPC. This way, the binder may remain free of these additional materials.
In order to satisfy these mechanical, chemical and thermal durability requirements for the OPC, then, a unique cross-linkable polymeric binder material must be obtained.
Generally, cross-linking polymers such as epoxy, phenolic resin, polyurethane, etc., has been known. For reinforced fiber plastics in the electronics packaging industry, for example, significant improvement in the glass transition temperature (T.sub.g) has been obtained by cross-linking with heat, radiation (e-beam, UV, X-ray, etc.), and/or moisture. However, for OPC applications, general cross-linking principals cannot be freely practiced because photoconductor components such as charge generation molecules (dye, pigment, etc.) and charge transport molecules are vulnerable to the heat, high-energy radiation and moisture used in the conventional cross-linking processes. Therefore, after cross-linking, these molecules may not exist in the cross-linked product in forms in which they are functional as charge generation or charge transport molecules. This is why prior attempts at cross-linking photoconductor binders have not been successful, whether for hole transport molecules such as hydrozones, arylamines, pyrazolines or triphenylmethanes, or for electron transport molecules, such as diphenyl sulfones, fluorenones, quinones, or whether the photoconductor is in a single or a multiple layer. All these attempts exhibit poor compatibility of the transport molecules in the cross-linked binders, resulting in undesirable photodischarge characteristics.
A fourth object of this invention is to provide a cross-linked binder for an OPC with superior adhesion to other polymer layers. This way, multi-layered OPC's may be made which do not separate too easily and come apart at the interface between the layers.
Among the conventional thermoplastic binders, polyvinyl butyral (PVB), is observed as the best binder for good dispersion and good film forming for many classes of photoconductive pigments in the applications of photoconductor technology. Still, the use of the thermoplastic PVB for phthalocyanine pigment in the single layer (+)OPC, doesn't show superior performance compared to the other conventional thermoplastic binders for photoresponse to the 780 nm laser diode, electrical stability, and environmental stability to heat and liquid toners. Also, the use of thermoplastic PVB as binder for the charge generation layer in the dual layer photoconductor, in general, exhibits poor adhesion due to the cohesive failure effect associated with the incompatibility between the binder of the charge generation layer (CGL) and the binder, usually phenylpolymers such as polycarbonate, polyester, polyimide, polystyrene, etc., of the charge transport layer (CTL).
This invention aims at a preparation method for such kinds of infrared-sensitive photoconductors using cross-linkable binder for long-life applications.